Molding mixtures containing quartz sand, clay, bentonite, coal dust and water are widely known in the preparation of casting molds. Coal dust is used in bentonite-containing molding mixtures as glossy carbon carrier for improving the surface quality of the casting, and also as an additive for reducing the burning on of the sand onto the cast object.
After the casting in bentonite containing molds containing coal dust additive and of cores new sand, it is known to add bentonite, coal dust and water to the removed, then in continuous recirculation reused sand mixture for supplementing the burned out coal dust, and the amount of the degraded bentonite. This is done by separate measuring and adding methods to refine the molding sand.
In addition to the aforementioned known advantages of using coal dust in casting, there are also a number of disadvantageous physical chemical and colloidal effects.
During our investigations we have found that the desirable solidity of sand mixtures that are reused after casting in circulation the bentonite containing molds and cores containing coal dust additive, can be readjusted only by using much greater amounts of bentonite than heretofore.
The reason for this is that due to unfavorable chemical reaction, the Na-cation exchange necessary in the coal dust additive containing molding sand for the formation of the maximal working capacity of the bentonite added while refining, can be formed only to a very small degree. This is so, on the one hand because of the time dependence of the reaction rate of the ion exchange, on the other hand because of the lack of suitable pH of the reused molding sand and of the necessary Na-ion-concentration.
We have found that the aforementioned essential and important preconditions cannot be endures by the traditional refining method due to the continuous addition of the coal dust additive, and due to its physical and especially dangerous chemical characteristics, that are so different from those of bentonite.
Coaldust cannot be suitably homogenized during refining, during the short mechanical stirring either by the added bentonite or by the reused molding sand due to the greater specific gravity and hydrophobic characteristics of the bentonite. In the casing mold and in cores prepared from molding sands the desired constant strength of material cannot be obtained.
The main problem is probably due to the especially disadvantageous physical, chemical and colloidal properties of the especially disadvantageous combustion by-products of the coal dust to bentonite.
Due to the heat in casting, not only the advantageous glossy carbon will be formed from the coal dust but at the same time fused slag and powdered ash are also formed in very significant amounts, of about 18-35 mass % as solid combustion by-products.
Their presence in the casting molds with bentonite binding and in the cores is very disadvantageous, because the fused slag contains substantial amounts of refractory granule attached to the surface of the cast object by which the required time and energy involved in cleaning the surface of the cast object is significantly increased.
The presence of powdered sand is also disadvantageous because the molding sand becomes continuously and quickly turned to dust. Thus, its ability to be formed deteriorates, its water requirement increases, along with its permeability and technological strength, increasing the amount of casting rejects.
The ashes of coal dust are disadvantageous not only for their mere presence, but mainly they concentrate strongly acidic compounds therein, which dissolve and dissociate well in the water of the bentonite-containing molding sand. Contrary to the alkaline chemical effect necessary for the advantageous Na-cation-exchange the pH of the electrolyte thus becomes reduced in the direction of the especially disadvantageous acidic pH range. The continuously increasing hydrogen ion concentration results in an especially unfavorable cation exchange to bentonite, whereby the swellability and working capacity will significantly be reduced or completely ceases.
The high sulfur content of the coal dust also results in some of the fundamental problems. As a result of the heat during casting, not only the advantageous glossy carbon forming hydrocarbon develop from the coal dust additive of the bentonite-containing molds and cores, but also significant amounts of SO.sub.2 gas are formed from the sulfur content. A large part of the SO.sub.2 is absorbed in the molds and cores themselves, while another part of the SO.sub.2 evaporates into the air space of the casting house and causes various labor hygienic and environmental protection problems, as well as acts as a corrosive.
The absorption of SO.sub.2 gas is very disadvantageous in the bentonite-containing casting molds and cores because under the conditions after the different oxidation reactions sulfurous acid or sulfuric acid is formed, which dissolve well in the water that is present, they dissociate well and ensure thereby the continuous formation of acidic pH within the casting molds and cores. In other words they increase the unfavorable hydrogen ion concentration and thus force a disadvantageous cation exchange to the sill bindable bentonite, as they transform it to hydrogen bentonite.
Thus they significantly decrease or completely eliminate the advantageous swellability and working capacity of bentonite, resulting in a reduction of the desired strength of casting molds and cores.